Family has a 2 year long text with 7 others, many happy memories, wish to keep it going. One participant got a new phone number and now doesn't get the text messages. The others still do. If a new text is created using the new number, all get the new messages. But not the old stuff.

I have tried to edit the contact info to replace the old number but can't. "Save" after changing the number pops a message "Invalid account selected (SyncML). Tried several things, including rebooting the phone, nogo so far.

If I understand this correct, your trying to see a text message with/from a new number but do not want to delete the old message string just edit the old number to the new one. but keep the string. I might be wrong but the phone thinks the new number is an error and states so. It may work if you just add the new number as a new contact and and leaving the old name there add a new one such as ( old ) Fred to ( new ) FredNew. You did say when the new number sends everyone gets the message. So to keep the old messages the new number needs to be added but with a different name..